The present invention is related to electron guns for producing bunched electrons and subsequently using those electron bunches to generate rf energy. More specifically, the present invention is related to an electron gun that uses an rf cavity subjected to a rotating and oscillating electric field at a given frequency for the production of bunched electrons and uses an output cavity for the production of a higher frequency and higher power oscillating electric field than that power and frequency in the input cavity.
The development of high-current, short-duration pulses of electrons has been a challenging problem for many years. High-current pulses are widely used in injector systems for electron accelerators, both for industrial linear accelerators (linacs) as well as high-energy accelerators for linear colliders. Short-duration pulses are also used for microwave generation, in klystrons and related devices, for research on advanced methods of particle acceleration, and for injectors used for free-electron laser (FEL) drivers. During the last few years, considerable effort has been applied to the development of high power linac injectors [J. L. Adamski et al., IEEE Trans. Nucl. Sci. NS-32, 3397 (1985); T. F. Godlove and P. Sprangle, Part. Accel. 34, 169 (1990).] and particularly to laser-initiated photocathode injectors [J. S. Fraser and R. L. Sheffield, IEEE J. Quantum Elec. QE-23, 1489 (1987); R. L. Sheffield, E. R. Gray and J. S. Fraser, Proc. 9th Int""l FEL Conf., North Holland Publishing Amsterdam, p. 222, 1988; P. J. Tallerico, J. P. Coulon, LA-11189-MS (1988); M. E. Jones and W. Peter, IEEE Trans. Nucl. Sci. 32 (5), 1794 (1985); and P. Schoessow, E. Chojnacki, W. Gai, C. Ho, R. Konecny, S. Mtingwa, J. Norem, M. Rosing, and J. Simpson, Proc. of the 2nd Euro. Part. Accel. Conf (1990), p. 606.]. The best laser injectors have somewhat higher quality beams than more conventional injectors such as in reference [J. L. Adamski et al., IEEE Trans. Nucl. Sci. NS-32, 3397 (1985)], but the reliability depends on the choice of photocathode material, with the more reliable materials requiring intense laser illumination.
The methods used to date are rather complex, cumbersome, expensive, and have very definite limits on performance.
The next generation of TeV linear colliders for high energy physics will require rf sources capable of 500 MW/m of rf power with a typical pulse length of 50 ns. This requires a 50 MW source with a corresponding pulse width of 1 xcexcs at a frequency between 10 and 20 GHz before pulse compression [R. Ruth, ed., Report of the Linear Collider Working Group, Proceedings of the 1990 Summer Study on High Energy Physics, Snowmass, Colo., Jun. 25-Jul. 13, 1990]. Because the cost of the rf sources will be a large fraction of the operating cost of the accelerator, there is a need for high-power microwave sources capable of multi-megawatt performance at high efficiency. To ensure that modulator costs do not become excessive, the potential driver should also be able to satisfy the above requirements working at a voltage of about 600 kV.
Considerable effort has gone into extending the frequency and power capabilities of xe2x80x9cconventionalxe2x80x9d klystrons [T. G. Lee, G. T. Konrad, Y. Okazaki, Masuru Watanabe, and A. Yozenawa, IEEE Trans. Plasma Sci., PS-13, No. 6,545 (1985); M. A. Allen et al, LINAC Proc. 508 (1989) CEBAF Report No. 89-001; M. A. Allen et al, Phys. Rev. Lett. 63, 2472 (1989)] to cope with the requirements of future linear colliders. At this frequency range, klystrons tend to become small and rf breakdown in the cavities and gaps becomes very difficult to avoid. The output power of the device is then constrained by the maximum electric field that the gap can sustain. As the frequency is increased the gap is reduced and so is the output power. In recent X-band klystron experiments at SLAC designed to produce 100 MW output power at 11.4 GHz, 52 MW was obtained with 1 xcexcs pulses at an efficiency of 30%. Output power was limited by breakdown in the output structures [G. Caryotakis, SLAC-PUB-6361 September 1993 (A)] and problems such as beam interception in the beam tunnels were also encountered.
Interest has increased in recent years in pursuing other methods of microwave generation oriented towards coping with the requirements of future TeV linear colliders. A group at the University of Maryland is pursuing an X-band gyroklystron amplifier [V. L. Granatstein et al xe2x80x9cHigh-power Microwave Sources for Advanced Acceleratorsxe2x80x9d, Am. Inst. of Phys. Conf. Proc. 253 (1991); W. Lawson, J. P. Calame, B. Hogan, P. E. Latham, M. E. Read, V. L. Granatstein, M. Reiser and C. D. Striffler, Phys. Rev. Lett. 67, 520 (1991); W. Lawson, J. P. Calame, B. Hogan, M. Skopec, C. D. Striffler, V. L. Granatstein, and W. Main, IEEE Trans. Plasma Sci. 1992; and S. Tantawi, W. Main, P. E. Latham, G. Nusinovich, B. Hogan, H. Matthews, M. Rimlinger, W. Lawson, C. D. Striffler, and V. L. Granatstein, IEEE Trans. Plasma Sci. (1992)]. At Novosibirsk, in the former Soviet Union, a significant advance has been made with the invention of the magnicon [Karlimer, et al, Nucl. Inst. and Meth A269 (1988), pp 459-473] which has produced 2.6 MW at 0.915 GHz with an impressive conversion efficiency of 76%. In the magnicon the proper adjustment of a focusing static magnetic field allows the electrons in the beam to maintain temporal phase coherence with the rotating modes contained in suitable microwave resonators. This results in long and efficient interactions i.e., longer cavities, which is an advantage over klystron and gyro-klystron cavities. In the United States, a harmonic experiment is currently being conducted at the Naval Research Laboratory (NRL). The input scanner resonator is driven at 5.7 GHz and power is extracted from a gyroresonant harmonic interaction (TM210 rotating mode) at 11.4 GHz [W. M. Manheimer, IEEE Trans. Plasma Sci. 18, 632 (1990); and B. Hafizi, Y. Seo, S. H. Gold, W. M. Manheimer and P. Sprangle, IEEE Trans. Plasma Sci. 20, 232, (1992)].
The described invention is a high power frequency multiplying device that utilizes a xe2x80x9cGatlingxe2x80x9d Micro-Pulse Gun (GMPG). The GMPG produces a number of electron bunches per rf period using a natural bunching process that results from resonant amplification of a current of secondary electrons in an rf input cavity. This natural bunching provides high-current densities (0.005-10 kA/cm2) in short-pulse (1-100 ps) beams, which when combined with a rotating mode, can produce many bunches per rf period and therefore can be used for frequency multiplication in an output cavity. The GMPG is an outgrowth of a simpler device, the Micro-Pulse Gun (MPG) [Patent Pending], that operates on the same fundamental principle but with only one bunch per rf period. Unlike thermionic or field emission devices which have a relatively short lifetime, the GMPG secondary emission process does not cause erosion or evaporation and therefore will have a longer lifetime. Furthermore, the natural bunch formation is a resonant process which is not prone to phase instability.
A system is described for producing a high-power high frequency microwave generator using a Gatling Micro-Pulse Gun. The system consists of five distinct components: (1) the GMPG which includes an output grid; (2) a post-acceleration section; (3) a radial magnetic compression section; (4) an output cavity; and (5) a beam collector. The system has been characterized in detail for: the transverse normalized emittance [xe2x80x9cThe Physics of Charged-Particle Beamsxe2x80x9d, I. D. Lawson, Clarendon Press, Oxford, (1977), p. 181], energy spread, and bunch expansion throughout the entire system. This is important for determining the output power and system efficiency.
The basis of the concept is a novel device to generate multiple, high-current density, micro-pulse electron bunches. The device is named the Gatling Micro-pulse Gun (GMPG). It utilizes the resonant amplification of electron current by secondary emission in an rf cavity, with pre-designated areas on one side of the cavity that are partially transparent to allow the transmission of output bunches. Multiple bunches are produced sequentially during an rf period by exploiting the unique properties of a rotating electromagnetic mode. The mode illustrated is TM010. This method allows frequency multiplication in an output cavity.
One application of the GMPG is high-power, high-frequency microwave generation. The narrow bunches are required for this application.
The final current density in the GMPG increases rapidly with frequency, namely as frequency cubed. The upper frequency will be limited by practical considerations such as required peak power, finite secondary emission time, secondary-emission current density for the input cavity, and breakdown in the output cavity.
The GMPG has been thoroughly characterized by finding the saturated current density dependence on the gap spacing, peak cavity voltage, resonant frequency and applied axial magnetic field. The peak particle energy emerging from the GMPG has also been characterized by finding its dependence on gap spacing, peak cavity voltage and frequency. Peak particle energy from the input cavity always corresponds to about, 75% of the peak rf voltage. Beam loading and frequency shift have been evaluated and can easily be tolerated. Setting up the required TM110 rotating mode in the input cavity of the GMPG has been established along with means to efficiently couple power into the GMPG without significant mode distortion. Absolute power requirements and the loaded Q of the GMPG have been found and are not restrictive. Breakdown in the input cavity has been examined and is not a problem. The beam emittance has been determined for various conditions of grid wire thickness, grid wire densities, axial magnetic field strengths, and magnetic scale length. While the presence of the output grids causes some emittance growth, the results are not significant for the intended application. Both rf and post acceleration field leakage through the grid region have been evaluated and shown to be insignificant. The GMPG mechanism minimizes emittance growth compared to a DC type gun. Resonant particles are loaded into the wave at near zero rf phase angle; thus, the resonant particles experience a much lower transverse kick from the grid wires. Also, by providing a 45xc2x0 radial focusing electrode after the second grid, the transverse field at the second grid is reduced significantly, which minimizes emittance growth. The only significant transverse emittance growth comes from the magnetic compression region, which only causes a reduction in the output cavity efficiency of about 3%. Grid heating has been shown not to be a problem. An input cavity design has been used which includes tapered waveguide for feeding in rf power and provides electric and magnetic beam focusing. In addition, various materials have been used for fabrication of the input cavity.
During post-acceleration, the transverse emittance growth and bunch expansion do not significantly affect the system performance. A design utilizing pulsed high voltage was used for post acceleration.
Magnetic compression is used to bring the bunches near the axis for injection into the output cavity. The most significant increase in transverse emittance occurs in this section. However, the loss of device efficiency is only a few percent.
The fourth component of the system is the output cavity. For good coupling, low transverse emittance growth (or high beam conversion efficiency) and high power handling, the TM040 mode was used. The TM040 mode locks at the output frequency and shows no mode competition. The fifth component is the beam collector which is also used for energy recovery.
With operation at an rf output power of 50 MW at 11.4 GHz, the resulting system efficiency is 59% without beam energy recovery and 75% with beam energy recovery. The system efficiency includes the input cavity efficiency, input driver efficiency (a 15 MW klystron at 2.85 GHz), output cavity efficiency, and the beam collector conversion efficiency. Breakdown in the output cavity appears to be manageable. One of the advantages of the GMPG over a klystron is that the bunch length is short compared to the rf period, which gives rise to higher beam to rf conversion efficiency.
The first component of the present invention pertains to the electron gun. The electron gun comprises an rf cavity having a first side with emitting surfaces and a second side with transmitting and emitting sections. The gun is also comprised of a mechanism for producing a rotating and oscillating force which encompasses the emitting surfaces and the sections so electrons are directed between the emitting surfaces and the sections to contact the emitting surfaces and generate additional electrons and to contact the sections to generate additional electrons or escape the cavity through the sections.
The sections preferably isolate the cavity from external forces outside and adjacent to the cavity. The sections preferably include transmitting and emitting grids. The grids can be of an annular shape, or of a circular shape, or of a rhombohedron shape.
The mechanism preferably includes a mechanism for producing a rotating and oscillating electric field that provides the force and which has a radial component that prevents the electrons from straying out of the region between the grids and the emitting surfaces. Additionally, the gun includes a mechanism for producing a magnetic field to force the electrons between the grids and the emitting surfaces.
The first component of the present invention pertains to a method for producing electrons. The method comprises the steps of moving at least a first electron in a first direction at one location. Next there is the step of striking a first area with the first electron. Then there is the step of producing additional electrons at the first area due to the first electron. Next there is the step of moving electrons from the first area to a second area and transmitting electrons through the second area and creating more electrons due to electrons from the first area striking the second area. These newly created electrons from the second area then strike the first area, creating even more electrons in a recursive, repetitive manner between the first and second areas. This process is also repeated at different locations.
The second component of the present invention pertains to the post-acceleration to high energy of the electron bunches from the first component, the electron gun. Outside the transmitting area of the electron gun an accelerating electric field provides the means to accelerate the electron bunches to high energy.
The third component of the present invention pertains to the means for decreasing the radial location of electron bunches. After post-acceleration, the electron bunches have to be prepared for the interaction in the fourth component, the output cavity. An external magnetic field which increases in strength along the path of the accelerated electron bunches is used as a means to decrease the radial position of the electron bunches so that they can be injected into the output cavity. The radial decrease is performed in such a way that the radial position of the bunches, after reduction, is equal to the radius at which the output cavity electric field is at a peak.
The fourth component of the present invention pertains to the means for producing coherent microwave radiation in a cylindrical output cavity. The driving source of energy comes from the electron bunches arriving into the output cavity, one every rf period of the output cavity radiation frequency. The intention is to force the bunches to couple near the peak of the axial electric field of the design mode. The electron bunches are allowed to pass through holes that are equally-spaced azimuthally in the output cavity walls (both front and back faces).
The fifth component of the present invention provides a means to collect the electron bunches and provide energy recover so as to produce a voltage to accelerate the initial electron bunches.
The present invention pertains to an electron gun. The electron gun comprises an rf cavity having a first side with multiple non-simultaneous emitting surfaces and a second side with multiple transmitting and emitting sections. The electron gun also comprises a mechanism for producing a rotating and oscillating force which encompasses the multiple emitting surfaces and the multiple sections so electrons are directed between the multiple emitting surfaces and the multiple sections to contact the multiple emitting surfaces and generate additional electrons and to contact the multiple sections to generate additional electrons or escape the cavity through the multiple sections.
The present invention pertains to an apparatus for generating rf energy. The apparatus comprises a mechanism focusing non-simultaneous multiple electron bunches. The apparatus also comprises an output cavity which receives non-simultaneous multiple electron bunches and produces rf energy as the non-simultaneous multiple electron bunches pass through it.
The present invention pertains to a method for producing electrons. The method comprises the steps of moving at least a first electron in a first direction at a first time. Then there is the step of moving at least a second electron in the first direction at a second time. Next, there is the step of striking a first area with the first electron. Next, there is the step of producing additional electrons at the first area due to the first electron. Then, there is the step of moving electrons from the first area to a second area. Next, there is the step of transmitting electrons to the second area and creating more electrons due to electrons from the first area striking the second area. Then, there is the step of striking a third area with the second electron. Next, there is the step of producing additional electrons at the third area due to the second electron. Next, there is the step of moving electrons from the third area to a fourth area. Then, there is the step of transmitting electrons to the fourth area and creating more electrons due to electrons from the third area striking the fourth area.